For example, Japanese Patent No. 4192396, which corresponds to US2001/0033477, describes a semiconductor module in which an upper arm and a lower arm are connected in series. Such a semiconductor module is known as a 2 in 1 package structure. Each of the upper arm and the lower arm has a semiconductor chip formed with a semiconductor power element, such as an insulated gate bipolar transistor (IGBT). In each of the upper arm and the lower arm, a heat sink is disposed on an emitter side of the semiconductor chip a metal block, and another heat sink is disposed on a collector side of the semiconductor chip. The semiconductor chip is sealed with a resin in a state where the heat sinks are exposed from the resin.
A heat sink that connects between the upper arm and the lower arm and has an intermediate potential is larger than the other heat sinks. The semiconductor chip of the upper arm is mounted on the large heat sink such that a front surface of the semiconductor chip faces upward. Also, the semiconductor chip of the lower arm is mounted on the large heat sink such that a rear surface of the semiconductor chip faces upward.
In such a semiconductor module, however, directions of mounting the semiconductor chips are not uniform. Therefore, an assembling process is complicated.
For example, Japanese Patent Application Publication No. 2006-140217 describes a semiconductor module in which a first semiconductor chip and a second semiconductor chip are mounted in the same direction. A heat sink (i.e., electrode plate) connected to a front surface of the first semiconductor chip and a heat sink (i.e., electrode plate) connected to a rear surface of the second semiconductor chip are connected to each other through a conductive member.
In such a structure, however, the conductive member is a separate member. Therefore, the number of components increases, resulting in an increase in the number of connecting portions between the components. Moreover, the manufacturing process is complicated. In addition, the heat sink connected to the front surface of the first semiconductor chip and the heat sink connected to the rear surface of the second semiconductor chip are extended to be close to each other. The conductive member is connected to the extended portions of the heat sinks to electrically connect between the heat sinks. Therefore, the area of the heat sinks increases, and hence the area of the semiconductor module increases.
For example, in Japanese Patent Application Publication No. 2006-222149, heat sinks are integrated in a connecting part for connecting between the upper arm and the lower arm. Further, the heat sinks are provided with projections as terminals (e.g., metal blocks).
In forming such a semiconductor module, first, a semiconductor chip formed with a semiconductor element for the upper arm is bonded with a heat sink and a semiconductor chip formed with a semiconductor element for the lower arm is bonded with a heat sink. Thereafter, the heat sinks with which the semiconductor chips have been bonded are fixed to the heat sinks integrated in the connecting part.
In a semiconductor module described in Japanese Patent Application Publication No. 2006-120970, a first semiconductor element and a second semiconductor element are mounted in the same direction. A heat sink connected to a front surface of the first semiconductor element and a heat sink connected to the rear surface of a second semiconductor element are connected to each other using a separate connecting part. Since the semiconductor elements are arranged in the same direction, the manufacturing process is simplified.
In such a semiconductor module, however, control terminals for controlling semiconductor power elements of semiconductor chips are fixed to a heat sink that is connected to the rear surface of the semiconductor chip until components are molded with a resin. The control terminals are cut after the molding. Therefore, even in a finished product, a hanging lead, which has the same potential as the rear surface of the semiconductor chip, exists adjacent to the control terminals. Depending on the potential difference between the control terminals and the hanging lead, it is necessary to ensure a creeping distance. Thus, it is difficult to reduce the size of the semiconductor module. Further, in a structure where the heat sink connected to the front surface of the semiconductor chip of the upper arm and the heat sink connected to the rear surface of the semiconductor chip of the lower arm are connected through a separate member, it is necessary to fix the components using a special means.
FIGS. 30A and 30B illustrate an example of a semiconductor module. FIG. 30A is a schematic plan view of a semiconductor module J10, before being molded with a resin. FIG. 30B is a cross-sectional view of the semiconductor module J10 taken along a line XXXA-XXXA in FIG. 30A. Although FIG. 30A is not a cross-sectional view, some portions are hatched for the sake of clarity.
The semiconductor module J10 includes semiconductor chips J11, such as a first semiconductor chip J11a for an upper arm and a second semiconductor chip J11b for a lower arm, leads J12, J13, J14, control terminals J15, and heat sinks J16, J17, such as lower heat sinks J16a, J16b and upper heat sinks J17a, J17b. The first semiconductor chip J11a and the second semiconductor chip J11b are formed with semiconductor power elements. The components J11a, J11b, J12, J13, J14, J15, J16, and J17 are integrally molded in a resin mold part J20.
The lower heat sink J16a is connected to the rear surface of the first semiconductor chip 11a, and the upper heat sink J17a is connected to the front surface of the first semiconductor chip 11a through a metal block J59a. The lower heat sink J16b is connected to the rear surface of the second semiconductor chip J11b, and the upper heat sink J17b is connected to the front surface of the second semiconductor chip J11b through a metal block J59b. 
Before being molded in the resin mold part J20, the leads J12 through J14 and the control terminals J15 are included in a lead frame J30. That is, the leads J12 through J14 and the control terminals J15 are continuous from a frame portion J31 of the lead frame J30. In this state, the leads J12 through J14 and the control terminals J15 are connected to the heat sinks J16a, J16b, J17a, J17b and the semiconductor chips J11a, J11b by soldering or the like. For example, the lead 12, which serves as a positive electrode lead, is connected to the heat sink J16a of the upper arm. The lead 13, which serves as an output lead producing an output potential, is connected to the heat sink J16b of the lower arm. The lead J14, which serves as a negative electrode lead, is connected to the heat sink J17b of the lower arm.
The heat sink J17a and the heat sink J16b are connected to each other through a connecting part J60 so that the heat sinks J17a, J16b have the middle potential. The control terminals J15 include control terminals J15a for the upper arm and terminals J15b for the lower arm. The control terminals J15a are connected to pads disposed on the front surface of the first semiconductor chip J11a through bonding wires J22a. Likewise, the control terminals J15b are connected to pads disposed on the front surface of the second semiconductor chip 11b through bonding wires J22b. 
The lead frame J30 has hanging leads J32a, J32b, J32c for restricting the components from inclining during the molding. The components of each of the upper arm and the lower arm are connected to the lead frame J30 at multiple corner portions thereof.
In FIG. 30A, the hatched portions with thin lines indicate connecting portions with the lower heat sinks J16a, J16b, and the hatched portion with thick lines indicate connecting portion with the upper heat sinks J17a, J17b. 
In such a configuration, a collector potential and an emitter potential (equal to the middle potential) of the semiconductor power element of the upper arm are respectively applied to the hanging lead J32b, which is adjacent to the control terminal J15a, and the control terminal J15a. Therefore, the voltage of 1000V or more may be applied to the portion b1 between the control terminal J15a and the hanging lead J32b when an inverter is operated. Likewise, a potential difference between the collector potential of the semiconductor power element of the upper arm and the middle potential occurs at a portion b2 between the hanging lead J32b and the hanging lead J32c. Further, a potential difference between the middle potential and the emitter potential of the semiconductor power element of the lower arm occurs at a portion b3 between the hanging lead J32c and the control terminals J15b. 
In the finished product, the hanging leads J32b, J32c, the control terminals J15b and the like are exposed from the resin mold part J20. Therefore, predetermined creeping distances need to be maintained at the portions b1, b2, b3 corresponding to respective potential differences.
As a result, a width of the semiconductor module J10, such as a dimension of the semiconductor module J10 in a right and left direction of FIG. 30A, increases. Accordingly, it is difficult to reduce the size of the semiconductor module J10. Further, since the connecting part J60 for connecting between the heat sink J17a of the upper arm and the heat sink J16b of the lower arm is the separate member, the components need to be fixed using a special means.